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Topical for this time of year (in the northern hemisphere at least), abscission referes to the rejection of plant organs, such as leaves in the autumn.

This occurs at an abscission zone, where hydrolytic enzymes reduce cell adhension. The process can be promoted by abscisic acid and inhibited by respiratory poisons, and is controlled in nature by the proportions and gradients of auxin and ethylene. Other hormones may be involved.

Abscisic acid is a terpenoid compound that is one of the five major plant horones. Althought it is synthesised in the chloroplasts, it occurs throughout the plant body and is particularly concentrated in the leaves, fruits and seeds.

It has a powerful grown inhibiting properties generally and also promotes leaf abscission and the senescence of plants and/or their organs, and induces the closing of the stomata and dormancy in seeds and buds.

Its effect is antagonistic to the plant growth hormones, and it is thought to act by inhibiting the synthesis of protein and nucleic acids.